This invention relates in general to plasma arc cutting torches. More specifically it relates to an improved electrode and insert cooling method for use in low current, high definition torches.
In plasma arc cutting of sheet metal and the like using air or oxygen as the plasma gas, it is common to use an insert of a high emissivity material such as hafnium or zirconium press fit into the bottom face of a copper electrode. A current is applied to the electrode. In a transferred arc mode of operation, a pilot arc is typically formed within the torch between the electrode and an adjacent nozzle. The arc then transfers to a workpiece in conjunction with a ramping up of the arc current to a full operating value.
In all electrodes of this type heretofore in commercial use, the insert is cylindrical and has a diameter of about 0.070 inch (17.8 mm) for torches carrying currents varying from 20 to 260 amperes. This value was chosen by Hypertherm, Inc., the assignee of the present application, in the 1980's during the development of a 260 ampere oxygen plasma cutting systems. It has remained the standard insert size ever since.
While the high emissivity of the insert is very desirable, in practice standard electrodes with these inserts exhibit an extremely short life when used at low current levels, e.g. 15 to 70 amperes. This problem is particularly severe for use in high definition torches where the current density is typically three to four times than that of a conventional torch and a strong vortex flow of the plasma gas is used to stabilize the location of the arc. For example, in an early test of a high definition torch using the standard insert size, when the operating current was 15 amperes the hafnium insert exhibited a wear depth of more than 0.030 inch after only 50 cycles of operation. This high wear rate is observed even when using the electrode wear reduction techniques described in U.S. Pat. No. 5,070,227.
It is therefore a principal object of the present invention to provide an electrode for plasma arc cutting torch that operates with a reactive plasma gas at a low operating current level and nevertheless has a useful life several times greater than that obtained with conventional electrodes for the same applications.
Another principal advantage is to provide an electrode and method of cooling the electrode that exhibits significantly improved wear and cut quality.
A further object is to provide an electrode with the foregoing advantages which is also less costly than conventional electrodes for comparable applications.